For the determining which atom loses an electron and which atom gains an electron, it is often very useful to consult the periodic table. From there, we are able to examine the chemical properties of an element; thus, determining their behavior in any chemical reactions. For this specific example between sodium (Na) and chloride (Cl), Na is element number 11 (meaning that it has 11 protons and thus 11 electrons to make a neutral atom) while Cl is element 17 (with 17 protons and 17 electrons when neutral). Na is also known as an alkali metal, which are all the elements on the left-most column while Cl is in the halogen group. It is well known that alkali metals and halogens react very well together to form an ionic interaction, such as NaCl, where one element donates an electron to become positive and another accepts that electron to be negative, resulting in a positive-negative charge interaction.
To determine which atom would lose an electron and which would accept one, we must focus on the amount of valence electrons on that atom (in other words, the amount of electrons in their outermost shell). Atoms like to attain the most stable energy level possible by having the maximum amount of electrons possible in its outermost shell but sometimes they can only do so through chemical reaction with other atoms. For the alkali metals, there is one electron in their outermost shell while halogens have seven. The full shell consists of eight electrons. As a result, it is much easier for halogens to obtain just one electron to complete the octet and for the alkali metal to lose that one remaining electron in its outermost shell so that their next outermost shell contains a full octet. This is what happens with Na and Cl. Na loses an electron to obtain a full octet while Cl obtains an electron to obtain a full octet. This creates Na+ and Cl-, which form an ionic interaction to become NaCl (neutral because + and - cancel each other out) commonly known as table salt.
The balanced chemical equation for the reaction between `Na` and `Cl_2` is:
`Na+1/2Cl_2 rarr NaCl`
When sodium chloride, NaCl, dissolves in water, it dissociates into sodium ions (`Na^+` ) and chloride ions(`Cl^-` ).
`NaCl ` `stackrel (rarr) (larr)` `Na^+ + Cl^-`
During the interactions between the atoms, the electron in sodium's outer energy level is transferred to the outer energy level of the chlorine atom. Sodium loses an electron to become a positive ion with a charge of +1. This is oxidation.
`Na rarr Na^+ + e^-`
On the other hand, chlorine gains an electron to form a negatively charged chloride ion. This is reduction.
`1/2 Cl_2 + e^- rarr Cl^-`
These two oppositely charged ions are then bounded by electrostatic force of attraction to produce crystals of `NaCl` .
If sodium and Chlorine were to form a bond, chlorine would be gaining the electron and sodium would be losing an electron.
Here's the reason why (simplified):
Sodium is in the first group of the periodic table giving it a +1 charge, chlorine on the other hand is on the 7th group of the periodic table giving it a -1 charge. Before I confuse you, I'll explain where the charges come from. All atoms combine in such a way as to achieve an octet or a pseudo noble gas configuration, to have an octet means to have 8 valence electrons. Sodium has 1 valence electron, it would be easier for it to lose the 1 electron then to gain 7 more, so when it loses the electron it has an octet and a +1 charge, chlorine on the other hand has 7 electrons and only needs 1 more electron to reach a noble gas state/an octet, when it gains the electron (which is negative) it receives a -1 charge.
Since sodium needs to get rid of its one electron to get an octet, and chlorine needs one more to get an octet, sodium will give away its one electron to chlorine and they both will have an octet and will form NaCl.
If we observe electronic configuration
Na ec is 1s2 2s2 2p6 3s1 = [Ne]3s1
Cl ec is 1s2 2s2 2p6 3s2 3p5 = [Ne]3s23p5
Na will get noble gas configuration [Ne] ( which means stability) when it loses 1 electron
while Cl will get outer shell octet when it gains 1 electron and thus noble gas configuration of [Ar].
Thus when Na and Cl combine Na loses electron while Cl gains electron.
Sodium has 1 valence electron so if it loses 1electron it will have a full outer energy shell. Chlorine has 7 valence electrons, so it wants to gain 1 electron so it has a full outer energy shell of 8 electrons. Therefore, Sodium loses 1 & Chlorine gains 1.